The invention relates generally to electric machines and, more specifically, to permanent magnet electric machines with reduced cogging effects.
Permanent magnet machines, such as a generator or an electric motor, typically include a rotor and a stator. For instance, in synchronous electric motors, the rotor may include permanent magnets to form magnetic poles. The magnetic poles are typically rotationally symmetrically disposed about the rotor. In these motors, the stator often includes a number of windings disposed between regularly spaced teeth. A time varying electric current passing through the windings creates a rotating magnetic field that interacts with the poles of the rotor. The magnetic field from the stator drives the rotor by repelling and/or attracting the magnetic poles of the rotor, thereby converting electrical energy into mechanical energy.
Synchronous electric motors often generate a torque as they rotate between adjacent teeth. This torque, which is generally undesireable and unrelated to the intended output torque, is referred to as cogging or a cogging torque. The torque generally results from the fact that the teeth of the stator often include ferromagnetic materials. Consequently, each magnetic pole of the rotor may be attracted to a number of adjacent teeth. As the rotor rotates, torque attributable to this attraction may vary as the edge of each pole crosses between teeth. During operation, this cogging torque may reduce the efficiency of the motor and cause vibrations that can adversely affect both the motor and driven loads, and that can, consequently, add maintenance costs to the motor, bearings and related equipment. This cogging torque also can degrade the quality of the product associated with the driven load.
While various approaches have been taken to reduce or avoid such cogging, these have not provided a suitable solution. For example, the number of teeth in the stator may be varied such that the number of teeth (and consequently the number of stator windings) is not an integer multiple of the number of poles. However, such solutions result in stators that may be difficult to wind or otherwise manufacture, particularly by machine-installation of windings in the stator slots. These designs may also result in electromagnetic dissymmetries leading to degraded machine performance. To some extent, the magnets of the rotor may be installed such that they are non-parallel to the stator teeth (i.e., skewed along their length). However, this approach limits the use of standard laminates and other simple and cost-effective structures for the rotor.
There is a need, therefore, for a new approach to the problem of cogging in permanent magnet motors. There is a particular need for an approach that can be used with motors having a number of stator windings that is a multiple of the number of poles, and that can be used with conventional unskewed permanent magnet rotors.